Field of the Invention
The present invention relates to a correction method for an image forming apparatus, for correcting distortion and uneven image density of an image during image formation of a two-dimensional image by the image forming apparatus, e.g., a digital copying machine, a multifunctional peripheral, or a laser printer.
Description of the Related Art
In an electrophotographic image forming apparatus such as a laser printer or a copying machine, there has been generally known a configuration to form a latent image on a photosensitive member with the use of a light scanning device configured to perform scanning with a laser beam. In the light scanning device of a laser scanning type, a laser beam collimated with the use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member with the use of an elongated fθ lens. Further, there is known multibeam scanning in which a laser light source having a plurality of light emitting points is included in one package so as to perform scanning with a plurality of laser beams simultaneously.
Meanwhile, in order to form a satisfactory image without uneven image density and banding (a striped pattern formed by light and shade in image density), it is desired that distances between scanning lines of which positions to be scanned with a laser beam are adjacent to each other in a rotational direction of the photosensitive member be equal to each other. However, the distances between the scanning lines are varied due to a plurality of factors described below. The distances between the scanning lines on the photosensitive member are varied by, for example, a fluctuation in a surface speed of the photosensitive member, or a rotation speed fluctuation of a rotary polygon mirror. Further, the distances between the scanning lines are also varied by a variation in angle of mirror faces of the rotary polygon mirror with respect to a rotary shaft of the rotary polygon mirror and a variation in intervals between light emitting points arranged on a laser light source. To cope with uneven image density and banding caused by such factors, there has been proposed a technology of correcting banding by controlling an exposure amount of the light scanning device. For example, in Japanese Patent Application Laid-Open No. 2012-098622, there is described a configuration in which a beam position detection unit configured to detect a beam position in a sub-scanning direction is arranged in the vicinity of the photosensitive member, and the exposure amount of the light scanning device is adjusted based on scanning distance information obtained from a detected beam position, to thereby make banding less noticeable.
As a configuration to adjust the exposure amount of the light scanning device, there are given a configuration to control the peak light intensity of a light emitting point and a configuration to control the light emission time of each pixel (PWM system). In the PWM system, there has generally been known a light scanning device including a PWM signal generating portion and a drive circuit. The PWM signal generating portion is configured to generate a control signal (PWM signal) for designating on-off of a light emitting point in accordance with image data (pixel value) of each image, and the drive circuit is configured to turn on or off the light emitting point in accordance with the PWM signal generated in the PWM signal generating portion. Similarly to, for example, Japanese Patent Application Laid-Open No. 2012-098622, as a configuration to make banding less noticeable by controlling an exposure amount, there is given a configuration to correct the positions of scanning lines by shifting image data in the sub-scanning direction in accordance with position information in the sub-scanning direction of each scanning line. In this configuration, the movement amount of an image center of gravity can be adjusted with a data value of image data to be added. When the data value is low, a pixel exposed to light in a small exposure amount is added, to thereby minutely move the image center of gravity. Meanwhile, when the data value is high, a pixel exposed to light in a large exposure amount is added, to thereby greatly move the image center of gravity.
In an electrophotographic image forming apparatus, banding is even caused by image positional deviation of from about 2 μm to about 5 μm. For example, in an image forming apparatus having a resolution of 1,200 dpi, the width of one pixel is 21.16 μm, and hence in order to correct the image positional deviation of from about 2 μm to about 5 μm, it is necessary to move an image center of gravity with a resolution of 1/10 pixel or less. Therefore, when the image center of gravity is moved by increasing or decreasing the exposure amount of each pixel or shifting image data, it is necessary to control each pixel with a small light emission intensity of 1/10 or less. For example, FIG. 20A is a chart for illustrating laser beam waveforms at a time when a light emitting point (semiconductor laser) is turned on through the PWM system. When the pulse width of a PWM signal is decreased, the light emitting point is not turned on completely (the output of a laser beam does not reach a predetermined setting value), and light intensity shortage occurs. Such phenomenon is referred to as a linearity defect because the light emission intensity does not change linearly with respect to a change in pulse width. FIG. 20B is a graph for showing a relationship between the pulse width of the PWM signal corresponding to a pixel value and the light emission intensity (light intensity of a laser beam) of a light emitting point. The light intensity of a laser beam changes linearly (in a linear function manner) in proportion to the pulse width of the PWM signal. However, it is understood that, of pulse widths of the PWM signal in sixteen stages, the pulse widths in the first to fourth stages (pixel values of from 1 to 4) form a light intensity unstable area in which the light emission intensity does not change linearly with respect to a change in pulse width, with the result that the linearity defect occurs.
Further, the banding correction system of moving an image center of gravity by adding image data as described above has a problem in that, when the linearity defect occurs, an error is caused in an exposure amount of image data to be added, and hence an error occurs also in the movement amount of an image center of gravity. Further, the light intensity decreases to also cause a decrease in image density, with the result that a density fluctuation occurs. FIG. 21 is an illustration of a state of a density fluctuation at a time when the linearity defect occurs when banding correction is performed. In an image area A, the image center of gravity is moved downward in the length direction of FIG. 21 through banding correction. Meanwhile, in an image area B, the image center of gravity is not moved because the image position is located at an ideal position. It is understood that, in the image area A, image density decreases to cause a density fluctuation due to the linearity defect. The details of FIG. 20A, FIG. 20B, and FIG. 21 will be described later.